Having cut my UNIX teeth on the JHU 11/45, I can tell you very much that 
it did have split I/D.    V6 supported split I/D for user mode programs. 
   The kernel originally wasn’t split I/D.   Version 7, if I’m recalling 
properly, did run the kernel split I/D on the 45 and 70.



------ Original Message ------
From "Kenneth Goodwin" <kennethgoodwin56@gmail.com>
To "Will Senn" <will.senn@gmail.com>
Cc "The Eunuchs Hysterical Society" <tuhs@tuhs.org>
Date 8/3/23, 5:05:31 PM
Subject [TUHS] Re: Split addressing (I/D) space (inspired by the death 
of the python... thread)

>At the risk of exposing my ignorance and thus being events long long 
>ago in history....
>And my mind now old and feeble...
>
>😆 🤣
>
>1.  I don't think the 11/45 had split I & d.
>But I could be wrong.
>That did not appear until the 11/70
>And was in the later generation 11/44 several years later.
>
>2. The kernel determined it by MMU type and managed it solely. The 
>assembler and loader always built the binary object file as the three 
>sections - instructions,  data and bss spaces so loading an object file 
>could be done on any platform.
>Programmers generally did not worry about the underlying hardware
>
>3. I don't recall if a systype style system call was available in v7 to 
>give you a machine type to switch off of.
>
>With something like that you could determine memory availability hard 
>limits on the DATA/bss side if you needed to.
>
>But that was also easily determined by a allocation failure in 
>malloc/sbrk with an out of memory error.
>
>If you really needed to know availability,  you could have a start up 
>subroutine that would loop trying to malloc ever decreasing memory 
>sizes until success and until out of available memory error.
>Then release it all back via free(). Or manage it internally.
>
>As I recall however vaguely,  there was an attempt to split the kernel 
>into two pieces. One running in kernel mode and one running in 
>supervisor mode in order to double the amount of available  instruction 
>and data spaces for the operating system. I recall playing around with 
>what was there trying to get it to work right.
>I was trying to support over 200 users on a pdp 11/70 at the time 
>running a massive insurance database system.
>
>On Thu, Aug 3, 2023, 4:35 PM Will Senn <will.senn@gmail.com> wrote:
>>Does unix (v7) know about the PDP-11 45's split I/D space through
>>configuration or is it convention and programmer's responsibility to
>>know and manage what's actually available?
>>
>>Will
>>
>>On 8/3/23 12:00, Rich Salz wrote:
>> > What, we all need something to kick now that we've beaten sendmail?
>> > How about something unix, ideally a decade old?
>>